1. Field of the Invention
The present invention relates to phase compensation of a DC-DC converter circuit.
2. Description of the Related Art
In general, when phase compensation is not performed on a DC-DC converter circuit, the stable operation thereof cannot be ensured. There has been widely known a method of connecting a resistor with an output capacitor in series as one of the methods of performing the phase compensation.
FIG. 3 is a circuit diagram showing a conventional chopper type boosting DC-DC converter circuit. For the phase compensation to ensure the stable operation, a phase compensation resistor 112 is connected with an output capacitor 111 in series, or an output capacitor provided with an equivalent series resistor is used, thereby providing a zero point. In order to perform sufficient phase compensation, it is necessary to set a frequency fz at the zero point in a frequency band for which the phase compensation is required. Here, assume that a capacitance value of the output capacitor 111 is given by Cout, a resister value of the phase compensation resistor 112 is given by RESR, the frequency fz at the zero point is expressed by the following expression (1).fz=1/(2Π×Cout×RESR)  (1)FIG. 4 is a timing chart of the chopper type boosting DC-DC converter circuit. In the chopper-type boosting DC-DC converter circuit shown in FIG. 3, a current flows through a rectifying device 110 when a switching element 104 is turned off, and the current does not flow through the rectifying device 110 when the switching element 104 is turned on. Therefore, a variation in current Ipk is caused in the rectifying device 110. Therefore, when the resister value of the phase compensation resistor 112 is given by RESR, a ripple voltage Vpk expressed by the following expression (2) is generated in an output voltage.Vpk≈Ipk×RESR  (2)In general, when the ripple voltage Vpk generated in the output voltage is large, normal feedback control is not performed, so the stable operation cannot be ensured. Therefore, in order to suppress generation of the ripple voltage Vpk in the output voltage, it is necessary to set the resister value RESR of the phase compensation resistor 112 to a small value. (See JP 07-274495 A (FIG. 1))
However, when the resister value RESR of the phase compensation resistor 112 is set to a small value, it is necessary to increase the capacitance value Cout of the output capacitor 111 in order to achieve sufficient phase compensation. Therefore, there is a problem in that a size of the output capacitor 111 is made large and a cost thereof is increased.